Advanced high-performance aircraft use angular rate and linear acceleration sensing for vehicle motion feedback in their flight control systems. These aircraft require either a Command Augmentation System (CAS) or a Stability Augmentation System (SAS) design for their primary flight control because their aerodynamic response to maneuvering commands is faster than the pilot is able to control. For example, the McDonnell Douglas F/A-18 Hornet is the first operational, high-performance tactical aircraft to use a digital fly-by-wire (FBW) primary flight control system where the flight control laws are programmed into digital computers. These computers are flight-critical and must be installed with sufficient redundancy for flight safety and mission requirements. Currently, high-performance tactical aircraft (as well as most commercial transport aircraft now in service) use hydromechanical flight controls with additional, limited-authority, electrically-powered stability augmentation systems (SAS). In this type of design, the pilot would have full control over the servovalve on the actuator and therefore also over the control surface. He can also use or disengage the SAS as desired and because the SAS is limited in authority, he can override its surface command in the event of failure. In many of these prior systems, the early, mechanical accelerometer used was the Kollsman pendulum (open-loop) design. In this device, the sensitive mass deflects two symmetric pendulae under load on their lever arms, and this motion is transferred by sector gears to a pointer on the dial face which indicates the current normal "g" load on the airframe. This device is a moment balance sensor with a pair of ratcheting indicators (pointers) which remain at the maximum and minimum (negative) reading sensed.
Advances have been made in the field of accelerometers and in one class of known electro-mechanical accelerometers, the sensitive mass is returned to its initial position (at zero or one "g") by a servo control drive. A measure of the force, such as the electrical current, required to drive the mass back to its initial setpoint transduces the measured acceleration to an electrical output. Other forms of electrical output devices, such as seismic or piezoelectric accelerometers, make accurate dynamic measurements of vibration and shock motions throughout the range of fractions of a "g" to thousands of "g's". In these solid state instruments, a relatively thin supporting member is attached to the sensitive mass, and the strain in this "spring" is transduced as the acceleration signal.
To satisfactorily perform the backup function to a primary FBW flight control system, the backup system should be technologically dissimilar (i.e. nonelectric) to eliminate such common mode failures as loss of electric power, generic computer faults, and various forms of electromagnetic interference. Research has shown that fluidics can protect these systems by providing a functionally redundant, nonelectric, stability augmentation control mode. For an integrated flight control system which has a fluidic reversion mode and in which the fluidic sensors are shared with the FBW primary system, the advantages can include lower levels of redundancy for the electronic controls, lower maintainance costs, less weight and improved survivability. Because the fluidic system is tracking the FBW system, any transient effects of reversion to the backup system can be minimized. Additionally, the signals from the fluidic sensors can be transduced to electrical outputs and also used by the primary system.